Molecular systems involving electron rich and poor π-conjugated substituents have been synthesized for energy band mixing applications. These molecular systems have been explored for use as: light-emitting semiconductors; n-channel and ambipolar semiconductors for organic thin-film transistors; active organic components for chemical biosensors; non-emissive organic electrochromics; and low-bandgap photovoltaics absorbing in the visible and/or near infra-red (NIR) regions of the electromagnetic spectrum. Low-bandgap photovoltaics absorbing in the visible and/or near infra-red (NIR) region have been pursued for the goal of designing bulk-heterojunction solar cells with high energy conversion. Donor-acceptor (DA) π-conjugated polymers that can be easily bandgap engineered via structural control and allow mechanical deformability are of interest to meet a demand for innovative high-performance flexible light-harvesting technologies. As opposed to the inorganic counterparts, DA π-conjugated polymers have the potential for low-cost scalability and high-throughput solution processing, particularly in solar cell's ultimate applications that require a large-area, for example in car and housing exterior surfaces, or require finely printed photoactive arrays as in portable electronic devices.
Although the donor-acceptor type macromolecules were introduced by Havinga et al., Synthetic Metals, 1993, 55, (1), 299-306) more than a decade ago, only recently have conjugated polymers been synthesized that exhibit good performance for use in photovoltaic cells (up to 5% of overall efficiency). Typically, DA polymers have displayed low power conversion efficiencies. This inefficiency can be attributed to two factors. The first factor being that the DA polymer's light absorption is generally limited to a small portion of the visible spectrum, typically the red region. The other factor is that the DA polymers have displayed low intrinsic charge carrier mobility in solid state devices.
There have been few disclosures of systems that achieve absorption over a large portion of the visible spectrum, and hence are essentially black. Nearly all polymeric systems that have been designed for full spectrum visible absorption have relied on having only a small number of broadly absorbing chromophores or have multiple chromophores with sharp absorption features evenly spread over the visible spectrum. The first solar cell systems that involve an all organic polymer semiconductor absorbing throughout the visible spectrum have only recently been reported, and even these systems do not absorb strongly (>50% of the peak absorption) over the entire visible range. (see Hou et al. J. Amer. Chem. Soc. published on the world wide web on Nov. 7, 2008, for alternating copolymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) and Wang et al., Appl. Phys. Lett. 2008, 92, 033307 for alternating copolymer poly[(2,7-dioctylsilafluorene)-2,7-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (PSiFDTBT))
The typical strongly absorbing DA polymers have displayed limited charge carrier mobilities due to a lack of favorable intermolecular interactions, specifically poor π-stacking due to large chain-to-chain distances, also called lamellar spacing, along with a lack of extended conjugation because of a low level of planarity of the polymer main-chains. High charge carrier mobilities are required for bulk-heterojunction solar cells as photo-generated excitons or geminate electron-hole pairs must undergo diffusion and dissociation processes within the active layer of the device and a rapid transport of the dissociated charges to collection electrodes. In absence of high charge carrier mobilities, the dissociated charges recombine before collection resulting in a device that displays poor solar energy conversion.
Hence an essentially black DA polymer system that absorbs strongly throughout the visible spectrum and displays good π-stacking with extended conjugation for high charge carrier mobilities remains a goal in the development of large area flexible light harvesting devices such as solar cells.